1. Field of the Invention
The present invention relates to an on-vehicle camera lens glass material used for an optical device such as an imaging lens mounted in an on-vehicle camera and to the on-vehicle camera lens.
2. Description of Related Art
Conventionally, in order to eliminate driver's blind spots while driving a vehicle, there has been proposed a method of installing an on-vehicle camera on a vehicle. Specifically, by installing a camera for imaging the rear and the side of a vehicle on the vehicle itself, an image taken by the camera is displayed on a position where a driver can check the image by his/her own eyes. Therefore, it is possible to eliminate the blind spots.
In view of a low price and light weight, an optical device made of plastic material or low refractive glass material is broadly used for an optical device such as an imaging lens mounted on the on-vehicle camera, as described in Japanese Patent Unexamined Publication No. JP-A-2006-11093.
By comparison between the materials in an optical performance of lens material, the glass material is more excellent than plastic material in various properties. Accordingly, in the imaging lens constituted of a plurality of lenses, it has been studied that a plastic lens and a glass lens are used jointly in order to decrease manufacturing costs and improve optical performance.
In such an on-vehicle camera, it is required to increase resolution and an imaging view angle of the camera in order to secure safety and improve recordability. However, it is difficult to configure an imaging lens suitable for the requirement by the plastic lens and the low refractive glass lens. Hence, in the imaging lens mounted in the on-vehicle camera, the configuration using high refractive optical glass can be considered.
In such an imaging lens mounted in the on-vehicle camera, unlike an imaging lens of general camera, there is a possibility that the lens is damaged or eroded by shock, wind pressure, and dust while vehicle is running. In addition, there is also concern about lens surface deterioration and metamorphosis that is caused by acid rain and chemicals such as a cleanser and a wax used at the time of car washing. Further, when oil, dust, and coarse particulates are attached on a surface of the imaging lens, sometimes the lens surface may be blurred as time elapses, and when a contamination is in contact with the lens surface for a long time, sometimes the lens surface may metamorphose.
In addition, when the camera is mounted on the front side of a vehicle body, there is concern about lens deterioration caused by heat generated from an engine and by nitrous oxide contained in gas exhausted from other vehicles which are running ahead of the vehicle. Further, the lens is frequently exposed under a severe environment where, for example, direct sun light including ultraviolet rays is irradiated on the lens for a long time. Particularly, when the lens has a wide-view angle, a first surface of the imaging lens has a larger exposed area. Therefore, the environment effect on the lens increases.
Accordingly, when the imaging lens mounted in the on-vehicle camera is made of plastic material or optical glass material used in a general imaging lens, there is concern that enough durability is not secured. The reason is that plastic material and glass material, which has low glass transition temperature suitable for a precision press forming process that enables mass production of an aspheric surface lens, have low hardness and are easily scratched. In addition, plastic material has lower ultraviolet resistance than glass material, and is easily deteriorated when ultraviolet rays are irradiated thereon for a long time.
In this case, it can be considered that a protective member made of an excellent glass material in chemical durability is disposed at the front of the imaging lens in order to prevent the lens from being directly exposed to outside environment such as wind or rain. However, when the protective member is installed, there is concern that the imaging view angle of the lens is restricted, and it becomes difficult to sufficiently enlarge the imaging view angle.